Electrical resistors



April 19, 1960 G. PEARCE 2,933,599

ELECTRICAL RESISTORS Filed May 3, 1955 E Invepnlor A Home y UnitedStates Patent 2,933,589 ELECTRICAL RESISTORS Graham Pearce, Crick, nearRugby, England,

to Painton and Company Limited, Northampton, England, a British companyApplication May 3, 1955, Serial No. 505,758 7 Claims priority,application Great Britain May 3, 1954 4 Claims. (Cl. 219-107 Thisinvention concerns improvements in or relating to rod-type electricalelements having end caps and electrical connection leads or tails, andmore particularly relates to electrical resistors of rod form having ametal end cap and a lead wire or tail connected to such cap to providefor the necessary electrical connections when the element is in use. 7When the term rod-type is used it should be understood that usually anelement of cylindrical form is referred to, but that cases may arisewhere other crosssectional forms are of interest and it should beunderstood that this invention is applicable thereto and need not belimited to elements of circular cross-sectional form.

-The invention is particularly, concerned with precision resistors foruse in electronic aircraft and radio apparatus of the kind comprising aceramic or other electrically non-conducting cylindrical core with aconductingfilm thereon, for example a metal or carbon film,theelectrical connections to this film being made through the medium ofmetal end caps furnished with wire leads or tails and provided on theends of the non-conducting core.

An accepted method of making resistors of the aforementioned kindcomprises placing the film-provided comes into an automatic machinewhich feeds them forwardly one after another into a cutting machinewhich, cuts through the film in a spiral path of predetermined form andpitch to give the required resistance. After the film has been cut inthis way to give the required resistance, metal end caps of cup-likenature are fitted one on each end of the element, the end cap having anintegral tail or having a tail soldered or welded thereto prior to theend cap being placed on the resistor.

In practice it is found that the resistance value of the element aftercapping is in many cases different from the value when held in thespiralling machine and this makes it almost impossible to produce closetolerance resistors (within 1% of nominal resistance) by this method.Although it is usual to apply a layer of material of low electricalresistance at the ends of the resistance element before it is placed inthe automatic spiralling machine, this does not completely avoid thechange of resistance referred to.

In prior attempts to cut the film of the resistor after the end capshave been applied and before the tails are attached to these end caps,applicant has experienced great ditficulty in subsequently attaching thetails to the end caps in such manner as to give a mechanically andelectrically sound joint without damaging the resistor or changing itsresistance value. The heating involved in attaching the lead by means ofhigh temperature solder (300 C. melting point or higher) is liable tochange the resistance value and also loosen the grip of the cap on thecore, and the use of such solder also leaves troublesome flux residues.On the other hand, the automatic machines available cannot handle thecutting of the films on resistors after end caps fitted with tails havebeen applied thereto.

It is therefore an object of this invention to provide 6 assignor asatisfactory method and means for attaching the tails to the end caps,whereby resistors can have their conducting films cut to the requiredform after the end caps have been applied to the cores, but before thetails have been connected to the end caps.

According to this invention, there is provided a method of manufacturingelectrical resistors of the type having wire tails, such methodcomprising taking an element of rod form; fitting to each end of thesaid element a metal end cap having in its end surface an inward recess;forming, between the two end caps, an electrical connection of thedesired resistance; applying a head formed on the wire tail to therecess in one of said end caps so as to position said tail with respectto said end cap; and then welding the head of said tail into the recessin the end cap.

According to a further aspect of this invention, there is provided amethod of manufacturing electrical resistors, such method comprisingtaking an element of rod form furnished with a conducting film; fittingto each end of the said element a metal end cap having in its endsurface an inward recess; cutting the film of the element to provide thedesired resistive path; forming tails with a head at one end to beconnected to the end cap; engaging the head of one of said tails in therecess in the end cap to which such tail is to be attached so as toposiof the resistor.

tion the tail with respect to the end cap; and then welding the tail tosuch end cap with the head of the tail pressed against the base of therecess in the end cap, which latter is supported on the opposite side bythe core Preferably the said recess in each of the end caps is ofapart-spherical form and the head of the tail is of a corresponding shapeso as to be capable of making a close fit with the recess over asubstantial area to ensure adequate metallic and electrical contactbetween the tail and the end cap. p

The radius of curvature of the said part-spherical recess in the end capis preferably approximately the same as the radius of curvature of thepart-spherical head of the tail, although it may, in some cases, bedesirable to producethese parts with initially slightly different radiiof curvature.

The recess in the end cap will preferably be centrally located and itwill be understood that in any case the inner surface of the end cap (atthe base of the said recess) will bear upon the adjacent end of the coreof the resistor and give a firm support for the end cap whilst the tailis being secured thereto. This firm support of the cap permits areliable weld to be made without the usual arrangement of a weldingelectrode on the inside of the cap which is precluded when the tail isbeing welded to the cap after the fitting of the latter to theresistance core. If no recess were provided and even were the end cappressed fully home, the end cap would be slightly resilient and wouldpreclude the obtaining of a reliable electrical connection between theend cap and the tail prior to welding; the Weld might therefore bedefective.

With this method as distinct from a soldering or brazing method, thewelding takes place in a fraction of a second and generates heat onlylocally and causes no disturbance to the resistance element.

In order that the invention may more readily be understood, one methodof carrying the same into effect is illustrated in the accompanyingdrawing which shows, in longitudinal section, the application of a tailto the end cap of a resistor.

In the drawing, the securement of the tail to the end cap is shown asbeing effected by the welding technique recently developed by theGeneral Electric Co. Ltd. The

drawing shows a generally cylindrical core 1 made such as of metal orporcelain and having a thin film 2 of carbon deposited on the peripheralsurface thereof. An end cap 3 formed as a brass pressing with a wallthickness of about .011" has been applied to the end of the core'linsucha manner as to be in electrical contact with the car-. bon film 2. Theend cap 3 is provided with a substantially central recess 4, the innersurface of which bears on the end face 5 of the core 1.

A tail 6 formed of S.W.G. copper wire is shown coaxially aligned withthe recess 4, this tail having a substantial head 7 formed thereon by anupsetting operation such as that employed in the formation of heads onpins and other similar metallic objects. The radii of curvature of therecess 4 and head 7 are substantially the same.

An annular electrode 8 is coaxially engaged with the outer end surfaceof the cap 3 and a further electrode (not shown) is engaged with thetail 6, the head 7 of the tail being located within (but out of contactwith) the annular electrode 8. On pressing the head 7 into engagementwith the recess 4 (by advancing the tail in the direction of arrow A),current flows and a weld is efiected between the tail 6 and the end cap3.

The recess 4 in the end cap 3 provides means for adequately positioningthe tail 6 with respect to the end cap whilst the securement of thesetwo parts co-axially together is being eifected and, by forming asubstantial head 7 on the tail, an adequate electrical contact over asubstantial area is provided between the cap and the tail and the dangerof forming an imperfect or high-resistance joint is greatly reduced.

Alternatively, however, the annular electrode 8 above described may bereplaced by clamping the second electrical lead to the outside peripheryof the end cap 3.

It will be appreciated that by this invention the end caps can besecured to the cores prior to the cutting of the films of the latter sothat this cutting operation can be effected to a very close toleranceand that the tails can subsequently satisfactorily be attached withoutaffecting the resistance of the resistor.

It will be obvious that the present invention is also applicable toresistors in which a resistance element is wound upon an insulating rod.Such resistance element may, for example, be resistance wire ormetalised glass fibre and may be Wound spirally upon the rod.

The rod, may, for example, be made of arceramic material such asporcelain or it may alternatively be made of sintcred alumina, e.g. thatsold under the registered trademark Sintox.

I claim:

1. A method of welding an electrical lead to a metallic end cap for aninsulating core carrying an electric resistive element which comprisesthe steps of forming an exterior recess on the end of said cap, fittingan end of said core within said cap until it abuts the inner wallsurface of said recess, forming a protruding head on said electricallead adapted to engage said recess, electrically connecting one of theelectrodes of a welding apparatus to said lead, electrically connectinganother electrode of said apparatus to a portion of the outer surface ofsaid cap spaced from said recess, and then pressing said head into saidrecess bearing against said core to electrically weld said lead to saidcap.

2. A method of welding an electric lead to a metallic end cap accordingto claim 1, including positioning said electrically connected lead inaxial alignment with the recess in said cap, electrically connectingsaid other electrode to the periphery of said cap in spaced parallelrelationship to said axially positioned lead and then moving said leadtoward said cap, pressing said head into said core-bearing recess toundeformingly spot-weld said head therein.

3. A method of welding an electric lead to a metallic end cap accordingto claim 2, including surrounding said axially positioned lead with anannular electrode to coaxially engage the periphery of said cap.

4. A method of welding an electric lead to a metallic end cap for aninsulating core carrying an electric resistive element which comprisesthe steps of forming a centrally positioned concave recess on the end ofsaid cap, fitting said cap onto said core with the raised inner wall ofsaid recess in abutting engagement with said core, forming a convex headon said electric lead complementary to said concave end, electricallyconnecting an electrode of a welding apparatus to said lead,electrically connecting another electrode of said apparatus to a portionof the outer surface of said cap spaced from said recess, and thenpressing said convex head within said core-supported concave recess toundeformingly spot-weld said lead to said cap.

References Cited in the file of this patent UNITED STATES PATENTS

